Rebels launched deadly attacks on Syria's military in a campaign increasingly targeting its air power, as Russia said it was "naive" to expect its traditional ally not to fight back.

Opposition forces captured the main air defence building in Albu Kamal on the Iraqi border, the Syrian Observatory for Human Rights said, adding reports suggested they had seized ground-to-air missiles.

The assault late Friday came hot on the heels of a rebel attack on the Abu Zohur air base in Idlib province in the northwest, where the Free Syrian Army said it downed a MiG warplane this week.